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Fork uses copy-on-write to speed forking. It creates a new environment, duplicates the page tables of the parent, and marks every non-shared entry in both environments as copy-on-write and read-only. Then, the page-fault handler in each process handles allocating new pages and duplicating them when a write occurs.[*]

Fork must also run through all the ExOS subsystems and notify them that a fork has occurred so that they can update their reference counts and other data.

Thomas Pinckney